The present invention relates to a method of and a system for controlling a pressurized hydraulic fluid supplied to hydraulically actuable torque transmitting couplings in an automatic transmission of an automotive vehicle.
Japanese Patent Application First (unexamined) Publication No. 61-130652 or U.S. Pat. No. 4,807,496 discloses an automatic transmission in which a line pressure solenoid is provided to generate a solenoid pressure which is supplied to a pressure modifier valve. The pressure modifier valve generates a modifier pressure in response to the solenoid pressure. This modifier pressure is supplied to a pressure regulator valve where the pressure regulation is effected on the modifier pressure to generate a line pressure which is supplied to hydraulically actuable torque transmitting couplings of the automatic transmission. According to the above-mentioned Japanese Patent Application First publication, the line pressure solenoid is controlled such that the line pressure varies in accordance with a predetermined pattern in response to an engine load in terms of a throttle opening degree. The line pressure versus throttle opening degree characteristic is designed to follow a torque versus throttle opening degree characteristic of an engine with which the automatic transmission is to be coupled. However, a change in engine torque due to a change in throttle opening degree is sluggish as compared to a quick change in line pressure due to the change in throttle opening degree. If the line pressure is too low during the transition phase, there occurs a slip in the hydraulically actuable couplings. If the line pressure is too high during the transition phase, there occurs a substantial shift shock on kickdown operation.
An object of the present invention is to improve the control of the above-mentioned kind such that the line pressure can follow well with an actual variation in engine torque upon occurrence of a change in load imposed on the engine, thus eliminating any slip or substantial shift shock on a kickdown operation.